Thermal energy storage devices can find use in many applications. For example, they can be used to preheat fuel cell coolant to heat a fuel cell stack, particularly on vehicles such as hybrid vehicles that include a fuel cell, to preheat the engine of a vehicle for rapid engine warm-up, to preheat a transmission for rapid transmission warm-up, and/or to heat a passenger compartment of a vehicle under conditions where the heat from the vehicle engine is not sufficient.
One way to heat the passenger compartment of a vehicle equipped with an internal combustion engine is to redirect some of the thermal energy carried away from the engine by the coolant system. Normally, coolant circulates between the engine and a radiator, absorbing thermal energy from the engine and rejecting the thermal energy to the environment via the radiator. To heat the passenger compartment, a portion of coolant from the engine is diverted to a secondary heat exchanger, where the coolant rejects its thermal energy to a stream of air, which is subsequently directed into the passenger compartment.
However, the supply of thermal energy in the coolant system often lags behind the demand for warmer cabin temperatures. In particular, if the engine is turned off in the cold weather, the thermal energy carried by the coolant system will soon dissipate into the environment. If the engine is then restarted and the heater turned on, cold air may actually be exhausted into the passenger compartment because the coolant is not warm enough to sufficiently heat the air stream passing through the secondary heat exchanger. Most people find this to be uncomfortable and undesirable.
To reduce this supply-demand problem, it is known in the art to equip the coolant system with heat batteries. Heat batteries are rechargeable devices which store the thermal energy generated during the operation of the engine for later release. Conventionally, heat batteries are included as an add-on to the standard coolant system, which, as described above, includes the radiator and the secondary heat exchanger as well as a circulation pump, a surge tank, and a series of conduits to connect the radiator, secondary heat exchanger, circulation pump and surge tank.
One commonly-used type of heat battery is a sensible heat battery. A conventional sensible heat battery is constructed by disposing an inner container within an outer container to define a space therebetween, and by filling the space with insulation. During the time the engine is operational, the circulation pump forces coolant through the coolant system and the inner container of the heat battery. When the engine is turned off, the circulation pump is deactivated and the coolant contained in the inner container of the battery is retained therein, the insulation preventing the thermal energy carried by this coolant from being rapidly exhausted to the environment. Upon subsequent start-up, the circulation pump forces the coolant in the battery out of the battery and through the remainder of the coolant system. See U.S. Pat. Nos. 5,558,055 and 5,765,511.
There are drawbacks, however, to adding sensible heat batteries to conventional coolant systems. Because coolant can only carry so much thermal energy in the form of sensible heat, the conventional sensible heat battery is quite large. To accommodate the addition of the heat battery to the coolant system, it may be necessary to remove or repackage other accessories, or enlarge the size of the engine compartment. Furthermore, the weight of the additional coolant needed to fill the heat battery (over that required to fill the coolant system) can degrade such weight-dependent performance characteristics as fuel economy. Moreover, heat batteries can be expensive to install and maintain.
Alternatively, it is known in the art to divert a portion of the coolant from the coolant system to the heat battery only when the engine is turned off. The diverted coolant is retained in the insulated heat battery to limit the loss of thermal energy. Prior to start-up, the coolant is returned to the cooling system. See U.S. Pat. No. 5,299,630.
However, it is believed that problems may arise when the coolant in the heat battery is pumped back into the coolant system. As the coolant displaces the air in the engine jacket, some of the coolant will become entrained in the exiting air stream. This coolant returns to the heat battery via a vent line, and remains there after the pump is deactivated. Consequently, with this heat battery there will always be some coolant which cannot be recovered for use in the coolant system.